CONCORD, N.H. — As New Hampshire again considers whether to legalize medical marijuana, neighboring states offer lessons about enforcement of the law, dispensaries and the complexities of implementing such a law.

The New Hampshire legislature has passed three medical marijuana bills in previous years, all vetoed by former Gov. John Lynch. This time, Gov. Maggie Hassan’s endorsement could tip the scales. Eighteen states and the District of Columbia already allow people who are sick or in chronic pain to legally buy and use medical marijuana, but laws vary widely.

A key concern of law enforcement officials is whether legalization would open the door for recreational users and people looking to profit from illicit distribution. Regardless of illness, marijuana could be used to treat conditions like severe pain or nausea, which could create an opening for recreational users to take advantage of the law.

But in neighboring Vermont and Maine, where medical marijuana has been available for years, police say that has not been their experience.

In Vermont, policing medical marijuana is “one more thing we have to deal with, but it’s not overwhelming,” state police Lt. J.P. Sinclair said.

He said he’s aware of only a half-dozen cases of patients or caregivers selling excess marijuana illegally since medical uses of it were legalized in 2004. Medical marijuana busts are not tallied separately from other marijuana crimes, he said, making it difficult to give an exact figure.

In Maine, where medical marijuana was approved in 1999, Hallowell Police Chief Eric Nason said his department sees burglaries related to prescription opiates and other drugs, but not marijuana. His department treats a dispensary in town like any other business.

But differences among marijuana laws may make comparisons with other states irrelevant, said Enfield Police Chief Richard Crate, who opposes such legalization. In Maine, for example, possession of small amounts of marijuana for non-medical reasons is only a civil infraction, so enforcement is not a high priority, Nason said.

Crate said he sympathizes with doctors and lawmakers that want to give terminally ill patients a measure of comfort, but the list of permissible conditions in the bill is too broad. He said New Hampshire police should not be burdened with distinguishing between medical and nonmedical users and legally and illegally grown pot, especially since budgets for police work are already spread thin.

Also unlike Vermont and Maine, the New Hampshire bill would allow patients with out-of-state medical marijuana cards to purchase from dispensaries. That’s bound to create challenges of its own, said Becky DeKeuster, who operates the Hallowell dispensary and three others in Maine.

“There’s such a patchwork here, with 18 states plus D.C. having different laws regarding possession limits and qualifying conditions I wouldn’t be comfortable treating patients from other states,” she said.

She said her dispensaries keep detailed records of where their marijuana is going. Because her business is still at odds with federal law that makes marijuana illegal, she said her businesses have to be “better than good” about self-regulating.

The New Hampshire proposal would also require those allowed to possess the drug to carry a registration card if they have marijuana. If they don’t, they could be fined and arrested. Caregivers and dispensary personnel would need federal background checks and could not have prior drug convictions.

The state registry called for under the law would contain information on patients, caregivers, dispensary employees and locations where marijuana is grown.

Concerns that the law will be manipulated are overblown, Rep. Donna Schlachman, D-Exeter, said at a recent hearing on the bill she introduced.

“There are lots of hoops to jump through. You cannot physician-shop and just start getting multiple prescriptions,” she said.

The program would generate no tax revenue, but as written, would not cost the state any money.

Dispensaries would need to be located 1,000 feet from any school and outside residential areas. They would also need to comply with local zoning laws, which have become an issue in other states. Cities and towns in Massachusetts, Maine and Vermont have sought to change zoning rules to block dispensaries from opening in their communities.

The issue was so contentious in Maine the legislature passed a law preventing municipalities from changing zoning requirements to keep out dispensaries, DeKeuster said.